PNG or JPG for an iPhone photo?
PNG keeps every pixel of the photo exactly and can be edited and saved again without losing quality, which suits a photo you are going to edit, or a form that asks for PNG. The cost is size: a 12-megapixel iPhone photo as PNG is often 10 MB or more, several times a JPG. For sharing, uploading or email, HEIC to JPG is the better choice; you can also switch to JPG above.
Safari opens HEIC by itself; in Chrome, Edge, Firefox and on Android the photo is decoded by a decoder that runs inside this page, so the photo is never sent anywhere.
What is kept
The photo keeps its full resolution and is turned the right way up. With "Keep the date taken" on, only the date and time the photo was taken are written into the PNG, so it sorts correctly in your gallery; location and camera details are never copied. A Live Photo gives its still picture.